Along with the rapid development and dissemination of computers and computer networks in recent years, various types of data, e.g., character data, image data, audio data and so forth, are digitalized. While digital data is resistant to deterioration with age and can be stored intact for good, it allows easy copying, thus causing serious problems in copyright protection. For this reason, the importance of the security techniques for copyright protection is rapidly increasing.
One of the techniques for copyright protection is “digital watermarking.” Digital watermarking is a technique for tracking down an unauthorized use of digital image data, audio data, character data and so on acquired by illegal copying, and is realized by embedding a name of a copyright holder or an ID of a purchaser in the digital data in a manner that it is not perceivable by humans. Since a digital watermark is exposed to various attacks, it must have resistance to attacks.
Attacks to digital watermarks are largely categorized into two types: one being a deliberate attack and the other being an indeliberate attack. The deliberate attack is an action taken by an attacker with an aggressive intention to eliminate or destruct the digital watermark. The indeliberate attack is an action taken by an ordinary user who performs general manipulation or editing on digital data, which ultimately causes elimination or destruction of the digital watermark. Examples thereof are irreversible compression, DA/AD conversion, or re-sampling of digital data. The embedded digital watermark must be resistant to either of the attacks.
As described above, fundamentally digital watermarks must be resistant to any kind of attacks and correct digital watermarks must be extracted from attacked digital data. However, in reality, it is difficult to realize digital watermarks resistant to all the attacks. Furthermore, in a case where an attack is repeated plural numbers of times, it is difficult to realize digital watermarks resistant to the plural numbers of times of attacks.
Meanwhile, digital data enables general users to perform manipulation and editing relatively easily. This is a significant advantage of digital data compared to analog data. However, performing manipulation or editing on digital data corresponds to the aforementioned indeliberate attack on digital watermarks. In other words, in a case where excessive manipulation/editing is performed on digital data or manipulation/editing is repeatedly performed on digital data, the digital watermark is often eliminated. On the other hand, it is necessary to allow manipulation/editing on digital data at liberty.
Take an example of image data, which is picked up by a digital camera and includes a photographer's ID as copyright information. It is preferable that the image data be manipulatable and editable by the photographer and that the digital watermark be extractable after the manipulation or editing is performed. Furthermore, in a case where image data including a digital watermark is encoded by an irreversible compression method, e.g., JPEG or the like, and stored, it is preferable that the digital watermark be correctly extractable from the stored image data. However, conventionally it has been difficult to realize such operation because these correspond to the aforementioned indeliberate attacks.